The present invention relates to signal processing equipment and methods for processing quadrature modulated color subcarrier television signals. More particularly, the present invention relates to improved control methods and circuitry enabling precision operation of comb filtering apparatus without introduction of any artifacts into the resultant signal on account of chrominance transitions in the vertical and temporal (time) domains.
Passive comb filtering techniques are known for separating chrominance and luminance components of a quadrature modulated color television picture signal. Such comb filters are typically implemented with single or multiple scan line and picture frame period delays. Since the phase of chrominance is in opposition from line to line and from frame to frame (in the NTSC signal format), the process of adding a present scanning line to a line which has been delayed by one scanning line period, or adding a frame to a frame which has been delayed by one picture frame period, results in phase cancellation elimination of the chrominance component, and resultant extraction of the luminance component from the composite video color signal. By a subtractive process the chrominance component likewise may be extracted from the composite video.
In quadrature modulated color subcarrier television signal formats, comb filtering is achieved by the process of adding information coming from a certain number of successive scanning lines. This manipulation is limited to spectral areas containing both the luminance and chrominance components (e.g. 2.3 to 4.2 MHz in the NTSC format) by means of bandpass filters. The full bandwidth luminance information is typically obtained by addition of the combed bandpass filtered component and a delay matched band-reject filtered component. The addition of signals coming from successive lines is carried out by taking the signal from the first line, for example, and multiplying the signal by a certain coefficient, and adding the signal from a second line as multiplied by a second certain coefficient, and adding the signal from a third line as multiplied by a third certain coefficient. In a typical situation, where the signal from line 1 is V1, the signal from line 2 is V2, and the signal from line 3 is V3, a standard comb filter arrangement is: EQU Y=1/4V1+1/2V2+1/4V3
(luminance in the vicinity of the subcarrier spectral area) and EQU C=1/4(2V2-V1-Vc) (chrominance).
In this example, the fractional values 1/4 and 1/2 are the coefficients of the comb.
Similar computations apply to temporal comb filters, where V1, V2 and V3 represent signals which are either undelayed (V1) or delayed by one picture frame period (V2) or by two picture frame periods (V3).
When compared to band pass filters and traps, passive comb filters work very well for separating chrominance and luminance, due to their wide bandwidth. However, the performance of passive comb filters breaks down when changes occur between lines or frames. When such changes appear, phase cancellation (averaging) from line to line or from frame to frame of the unwanted component does not perfectly occur. Instead, artifacts such as chroma blurring and horizontal dots in the luminance at the chroma subcarrier frequency are generated by the comb filtering process and may be objectionably visible to the viewer, particularly as the bandwidth of television displays has increased to include frequencies lying well above the subcarrier frequency (3.58 MHz in the NTSC system).
A number of proposals have been presented in the prior art for changing the comb filter structure or operation during transition conditions in an attempt to avoid the unwanted picture artifacts otherwise produced. In essence, the prior approaches have been either to alter the structure of the comb filter by on-off switching operations and/or to substitute a trap or other bandwidth limiting filter in place of the comb filter for the interval in which comb filter separation of chrominance/luminance breaks down. These prior attempts to make otherwise passive comb filters adaptive at vertical chroma transitions have not achieved a satisfactory solution to eliminate unwanted picture artifacts while maintaining high bandwidth characteristics of the comb filter, as will now be explained in greater detail.
One drawback of a trap or notch filter is that it is unidimensional, which is to say it only processes information occurring at the line scan rate. On the other hand, a comb filter may be operated in the horizontal and also in the temporal (picture frame to picture frame) domains.
One prior proposal is set forth in the Rossi U.S. Pat. No. 4,050,084. This patent describes a digitally implemented two scanning line delay comb filter for putting out a combed chroma component and a combed luminance component when there is no change of chrominance in the vertical direction (from line to line). The Rossi comb filter generates a combed chrominance component in the form of: ##EQU1## C.sub.c being band pass filtered, and generates a combed luminance component in the form of: EQU Y=V.sub.2 -C.sub.c
where V1 corresponds to the undelayed picture signal at the vicinity of the chroma subcarrier (3.58 MHz in the NTSC system), V2 corresponds to one scan line period delayed video, and V3 corresponds to two scan line period delayed video.
When the Rossi system detects a vertical amplitude transition in the chroma between two adjacent scan lines, Rossi's chroma comb filter system thereupon switches to a 0 V1+1/2 V2-1/2 V3 configuration for the vertical chroma transition interval at the first line thereof, and then switches to a -1/2 V1+1/2 V2+0 V3 configuration for the vertical chroma transition interval at the second line thereof. This reconfiguration of the Rossi comb filter is carried out in real time by manipulation between zero and one half amplitude coefficient values for the V1 and V3 terms of the comb during the transition. Thus, Rossi's progressive adaptation of the comb filter structure by coefficient manipulation enables it to avoid the chroma transition and the artifacts otherwise produced. In other words, the Rossi comb filter is adaptive in the sense that in the absence of a vertical chroma transition the output is combed on the basis of three scanning lines. When a vertical chroma transition occurs, combing reverts to a two scanning line basis, with information being combed coming during the first line of the transition from the second and third lines, and with information being combed coming during the second line of the transition from the first and second lines. One of the drawbacks of the Rossi system is that if there are multiple chroma transitions within three adjacent scan lines, the Rossi comb filter logic collapses, and that system switches to a notch filter for the duration of the trouble. That is to say, if V1 is different from V2 and V2 is different from V3, then the Rossi system switches from comb filter processing to a low pass filter in the luminance path and to a band pass filter (notch) in the chrominance path.
A significant drawback of the Rossi system is that it is controlled only upon detection of changes in chroma amplitude. The control signal relied upon by the Rossi system employs the difference of the rectified chroma from line to line. If, for example, a color phase shift (change in hue) occurs between two lines and it is not accompanied by a comensurate amplitude shift, the Rossi system is not capable of switching off the comb filter, and horizontal dots appear in the luminance in one or more scan lines of the picture.
A further drawback of the Rossi system is that it makes use of hard switching between the three operational configurations. There is no proportional or gentle switching between the three modes, and the switching transitions are abrupt.
Another comb filter system in the prior art was developed by Barco Electronic n.v., Noordlaan 5 Industriezone, B-8720 Kuurne, Belgium and was included in color television decoding apparatus introduced into the United States in about 1981. The Barco system was similar to that described in the Rossi patent, in that combing was switched from three lines to two lines in a manner that attempted to skirt the chrominance vertical transition. The Barco apparatus required an additional scan period delay line in order to provide a one line period look ahead or advance warning that a vertical chroma transition was imminent. Once a transition was detected on a look ahead basis, the Barco apparatus changed the coefficients of the comb on a step function (yes or no basis) by switching from the three line-based configuration to two two-line-based comb filter configurations during the line blanking interval for the lines having the detected chroma vertical transition. For the scan line preceding the chroma vertical transition the Barco comb filter coefficients would be: EQU Y=1/4 V1+1/2 V2+1/4 V3
(luminance comb wherein V1, V2 and V3 are bandpass filtered and Y refers to luminance components in the vicinity of the subcarrier energy spectral area). At the first line of the transition, the Barco comb filter coefficients are changed to: EQU Y=0 V1+1/2 V2+1/2 V3.
At the second line of the transition, the Barco comb filter coefficients are changed still further to: EQU Y=1/2 V1+1/2 V2+0 V3.
In this way, it is seen that the Barco filter structure is reconfigured by manipulation of coefficient values to "skirt around" the transition and thereby avoid the characteric picture artifacts otherwise generated by imperfect combing at the transition.
One immediately apparent drawback of the Barco approach was that an entire scan line of video information was necessarily modified as a result of switching during the horizontal blanking interval, even though only a very small portion of the scan line was subject to degradation by the presence of a vertical chroma transition. As a result, in some situations high frequency luminance components became offset spatially by one line with diagonal transitions taking on a visible raggedness or step effect.
The Barco system had the same drawback as the Rossi system in that it made use only of information in the chroma bandwidth in the vicinity of the color subcarrier in order to control the switching action.
Co-inventor Faroudja's prior U.S. Pat. No. 4,179,705 describes a method for switching comb filter apparatus. Essentially, the '705 patent describes a method to switch off the comb filter and replace it with a low pass filter in the luminance path and a band pass filter in the chrominance path in the presence of a vertical transition. The vertical transition was detected by looking at the differences in chroma energy on a line to line basis. While this system worked reasonably well, it had a time constant (slight delay) during which to make a control decision and suffered from the frequent situation that the chroma picture information is a very weak source of information upon which to make a decision concerning chroma processing. The slight delay in the control led to poorly defined (fuzzy or soft) chroma transitions during switching. Reducing the delay led to excessive control circuit implementation costs. Thus, the system either cost too much, or it let two or three horizontal dots get through to the display screen before the trap was switched in. Also, while this system worked well under test signal conditions, it proved inefficient in processing real picture signal content.
Mr. Faroudja's more recent U.S. Pat. No. 4,240,105 performs the same switching operation as was described in his '705 patent, but in response to different, additional control information. The system described in the '105 patent makes use not only of the chroma difference in the vertical domain but also the low frequency luminance difference which is statistically highly correlated to simultaneous chroma transitions and which provides a much stronger, more robust signal upon which to develop a switching control signal. However, while the switching based on the luminance transition was faster than the prior approach, the process of switching to a notch filter or trap in lieu of the comb filter structure during detected chroma transitions led to visibly soft transition edges: i.e. the reduced bandwidth resulting from the trap caused the picture to lack sharpness at the chroma transition.
In summary, each of these prior approaches had significant drawbacks. Substitution of low pass and band pass filters in lieu of the comb filter introduced blurring and ringing artifacts into the luminance and chrominance paths during the transition interval. Switching operations introduced transients into the picture which were not easily removed except at substantial circuit complexity and implementation expense. Switching during the blanking interval masked the switching transients problem, but substituted different scan line video composites on a line by line basis, rather than only during the chroma vertical transition interval. Furthermore, these prior systems did not separate adequately the chroma and luminance components during chroma vertical transition intervals occurring over more than one line period and occurring in a non-linear fashion.
Moreover, the adaptation of prior comb filter structures was limited to step function transitions of coefficients, such as between zero, one quarter, and one half unity values. No attempt was made to derive or use continuously and smoothly variable coefficients in a range between zero and unity, based upon the particular characteristics of each chroma vertical transition actually encountered in real television pictures. And, the step function switching of coefficients was abrupt, and necessitated care in design, and circuit complexity and expense in implementation in order to minimize switching transients.
A hitherto unsolved need has therefore arisen for improved control methods and circuitry enabling more precise control and adaptive operation of comb filtering apparatus in quadrature modulated color subcarrier television decoding apparatus which avoids introduction of any artifacts into the resultant signal on account of chrominance transitions in the vertical or temporal domain, irrespective of the particular characteristics of the transition (whether in the amplitude or phase parameter, and whether linear or non-linear in either parameter).